Pinion shims are used on all vehicles that come from the factory with leafsprings that are lowered or where a 4Link has been installed.  They are used to fine-tune the pinion angle to eliminate vibration.  They come in 2, 4, and 6 degrees.  Think of pinion shims as necessary to “align” your rear-end.  The driveline angle should be slightly negative, so when it is under power, it lifts to be straight in line and eliminates vibration.